Signature analysis is a method of determining the location and/or nature of a fault in an analog or digital system, like a component or a circuit, by input of test sequences and inspection and measurement of the resulting output sequences called signatures.
Several options are available for testing the analog and digital systems, and the measurement of the signatures can e.g. be made by digitizers and oscilloscopes. An oscilloscope is a digitizer with a screen for presenting an image of the measured signature on a display.
Digitizing or digitization is the representation of an object, image, sound, document or a signal (usually an analog signal) by a discrete set of its points or samples. The result is called digital representation or, more specifically, a digital image for the object and a digital form for the signal.
Digitizing can simply mean capturing an analog signal in digital form. A digitizer samples a waveform and transforms it into discrete values with an analog input path and with an Analog-to-Digital Converter (ADC). A digitizer can e.g. transform a measured voltage in digital form.
With a digitizer, standard oscilloscope measurements, like voltage, current, time and frequency, can be performed. Oscilloscopes can see the waveforms present at various points of a circuit and are excellent for looking at waveforms, and waveform shapes, which are shown on a display of a screen.
An oscilloscope allows observation of constantly varying signal voltages, usually as a two-dimensional plot of one or more signals as a function of time. Non-electrical signals (such as sound or vibration) can be converted to voltages and displayed. Oscilloscopes are used to observe the change of an electrical signal over time, such that voltage and time describe a shape which is continuously graphed against a calibrated scale. The observed waveform can be analyzed for such properties as amplitude, frequency, rise time, time interval, distortion and others. Modern digital instruments may calculate and display these properties directly.
An oscilloscope measures voltage on a momentary basis and can also present the value of the voltage and how it has changed in time, i.e. historically. From this history, it can be seen if the change is repeated and if so, how often. The interval of the changes constitute the frequency of the voltage and can be directly calculated by software in the oscilloscopes by measuring the time between the repetitions.
Analog Signature Analog signature analysis is an electronic component and circuit board troubleshooting technique which applies a current-limited Alternating Current (AC) across two points of an electronic component or circuit and the Analysis relies on a change in electrical characteristics to detect problems on a circuit board. The resulting current/voltage waveform is shown on a signature display using vertical deflection for current and horizontal deflection for voltage. This unique analog signature represents the overall health of the part being analyzed. By comparing the signatures of known good circuit boards to those of suspect boards, faulty nets and components can be quickly identified.
The usual waveform of an AC power circuit is a sine wave, but also other waveforms can be used in certain applications, such as triangular or square waves.
A waveform is the shape and form of a signal such as a wave moving in a physical medium or an abstract representation. In many cases, the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term ‘waveform’ refers to the shape of a graph of the varying quantity against time or distance. An oscilloscope can be used to pictorially represent a wave as a repeating image on a display of a screen. By extension, the term ‘waveform’ also describes the shape of the graph of any varying quantity against time.
A square wave is a non-sinusoidal periodic waveform (which can be represented as an infinite summation of sinusoidal waves), in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. The transition between minimum to maximum is instantaneous for an ideal square wave, which, however, is not realizable in physical systems. Square waves are often encountered in electronics and signal processing. Its stochastic counterpart is a two-state trajectory. A similar but not necessarily symmetrical wave, with arbitrary durations at minimum and maximum, is called a rectangular wave (of which the square wave is a special case).
A triangle wave is a non-sinusoidal waveform named for its triangular shape. It is a periodic, piecewise linear, continuous real function. Like a square wave, the triangle wave contains only odd harmonics, due to its odd symmetry. However, the higher harmonics roll off much faster than in a square wave (proportional to the inverse square of the harmonic number as opposed to just the inverse).
In this connection it is mentioned that Peak-to-peak voltage, Vpp, is a voltage waveform which is measured from the top of the waveform, called the crest, all the way down to the bottom of the waveform, called the trough. So peak-to-peak voltage is just the full vertical length of a voltage waveform from the very top to the very bottom. A peak-to-peak voltage is encountered in many environments. A voltage should not be assumed to be a peak-to-peak unless directly specified as that. For example, the voltage should say at the end, Vpp. For example, 60 Vpp. There is another type of voltage called the peak voltage. Sometimes peak voltage and peak-to-peak voltage are confused with each other. The peak voltage is exactly half of the peak-to-peak voltage waveform if it is symmetric with respect to the zero level, i.e. the signal has no direct voltage component.
Analog Signature Analysis relies on a change in electrical characteristics of the electronic component or circuit board to detect problems in them. The change is between the input test sequence and the resulting output sequence called signature. This resulting analog signature is unique for each particular type of a well working (“good” or “healthy”) component, like e.g. a resistor, capacitor or diode, known to be functioning. Any deviation from the normal signature strongly suggests the existence of a fault in that component.
The resulting signature is presented as a current/voltage waveform shown on a signature display using vertical deflection for current and horizontal deflection for voltage. This unique analog signature represents the overall health of the part being analyzed. By comparing the signatures of known good circuit boards to those of suspect boards, faulty nets and components can be quickly identified.
Power-off testing is often necessary in testing electronic components due to uncertainty as to the nature of the failure. When the electronic components can be further damaged by applying power it is necessary to use power off test techniques to safely examine it.
In analog circuit testing, a circuit can be characterized by various voltage levels or current levels at certain test points or, in the case of digital signature testing, also by the frequencies of certain signals at those test points.
Voltage or current level measurement is also used in connection with digital circuits to determine that the circuit is at least operating in some manner. In most situations, in which digital circuits are being tested, digital signals are available at various points in the circuit, which indicate the operation of the circuit.
In existing methods for testing of digital logic boards, a test sequence can be generated that will exercise the logic circuitry on a board under test and compare, at each node of the logic circuitry of the board under test, the response of the board under test with the corresponding node response of a known good board. The known correct responses are stored in a memory and fetched therefrom for comparison of the measured node responses of the board under test. The resulting signal response is individual for each node and test point and is measured on both sides of the test point.